This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/880,959 filed Jan. 18, 2007, the entirety of which is incorporated by reference.
The present application is generally directed to making pulp and is more specifically directed to screen assemblies for pulp digesters.
Wood chips and other cellulosic fibrous material are treated in digesters to chemically separate fibers in the chips and material by, for example, removing lignins. A digester is a vessel in which wood chips are treated with heat, liquid, and chemicals to convert the chips to pulp. A continuous digester vessel is typically an upright cylinder with an upper inlet to receive chips in a continuous flow. The chips flow slowly through the digester vessel, 100 to 300 feet tall (30 to 100 meters) in a generally downward direction.
As the chips move through the continuous digester, the lignins binding fibers together in the chips release the fibers and the chips are converted to pulp. The pulp is removed through a bottom outlet of the digester. Chips are continually added to a continuous digester while the chips already in the digester vessel are processed and pulp is discharged from the bottom of the vessel. In a batch digester, chips are first loaded in a vessel, the loaded chips are processed as a batch and thereafter the processed chips are discharged to empty the vessel. In a batch digester the chips tend to remain in substantially the same location in the vessel.
Chemicals, e.g., cooking liquor, in a digester process the chips, cause lignins to unbind fibers and convert the chips to pulp. The chemicals are included in cooking liquor that is continuously pumped into and out of batch and continuous digesters. Screen plates are used in conventional digesters for the production of chemical cellulose pulp, e.g. kraft pulp, for both continuous and batch digesters. Screen plates are filters that allow liquor to be extracted from a digester but prevent the extraction of fibrous material. Screen plates are generally arranged around an inner circumference of a digester. An inner surface of the plate is exposed to the chip slurry in the digester and an outer surface of the plate forms a wall to a liquor extraction chamber. The screen plate may have multiple rows of narrow slots through which liquor (but not fiber) is extracted from the chip slurry and flows into the extraction chamber.
The slots in screen plates tend to clog or plug with fibers and have been a source of a decrease in pulp process quality. Various types of slot designs have been developed to reduce the tendency of clogging and plugging. For example, orienting the slots diagonally to the vertical axis and horizontal planes of the digester has been found to reduce clogging and plugging of slots. See U.S. Pat. No. 6,165,323. However, clogging and plugging of the diagonal slots still occurs and there continues to be a long felt need for devices that further reduce the tendency of slot clogging and plugging.
A concern has arisen that chips in a digester clog the slots of a digester screen. Slots are narrow to block chips from being withdrawn from a digester along with the cooking liquor. While narrow, there is a risk that chips become logged in slots. This risk is relatively large with vertical slots in a continuous digester where chips move in the same direction of the slots. This risk is decreased with diagonal slots in which chips move vertically and at an angle with respect to the slots. As chips move across the diagonal slots, the chips may catch on the slots and clog the slots.
There is a long felt need for slots, especially diagonal slots, in a screen plate that have reduced risk of being clogged or plugged by chips. The need arises from the difficulties that occur when chips clog slots and prevent the flow of cooking liquor through the screen and out of the digester. While the need is greatest with respect to continuous digesters, there is also a need for clog free slots in screen plates for batch digesters, especially for diagonal screen plates.